But now she's revealing more of the tricks she used to pack on the weight to play a woman who just had her third baby in the Jason Reitman-directed film.
"I ate a lot of everything but my favorite thing, my drug of choice, is potato chips," Charlize, 42, said on "The Ellen DeGeneres Show" in a video posted to EllenTube on April 25.
"[I had chips] basically everywhere," she added. "I had a bag in my car, a bag in the bathroom, a bag in the kitchen, a bag on the couch, a bag in my trailer. Everywhere I went, there was just a bag."
Charlize previously told "Entertainment Tonight" while promoting the film that she also scarfed fattening carbs like mac and cheese to pack on the pounds. "I would literally wake up at 2 in the morning and I'd have a cup of cold macaroni and cheese just next to me," she said. "I would wake up and I would just eat it… I would just, like, shove it in my throat. It's hard to maintain that weight."
Charlize admitted that she wasn't required to gain weight for the role but said she wanted to do it so that she could put herself in the right frame of mind to embody this struggling mother. "I feel like I had to do it for the film if I was going to go and play a mother who was having her third child," she told Ellen. "I felt a responsibility to do that and I feel like, as an actor, I wanted to feel as much as I possibly could."
When she bulked up to play serial killer Aileen Wuornos in 2003's "Monster" — a performance that won her an Oscar — she had a much different experience, she admitted, explaining that she not only gained less weight but had much less trouble losing it.
"I was younger," Charlize told Ellen, noting that she was around 27 at the time. "I just didn't snack for a week and I was back into shape. It was insane. That doesn't happen anymore."
Charlize — who has two kids through adoption — empathized with moms who gain weight during pregnancy then have to put up with people who criticize their figures. "They get pregnant and they gain all that weight and it takes them a year and a half to lose it and if they don't, they get judged," she lamented.
When it came time to slim down after "Tully," Charlize told Ellen, "It was brutal." And not just the working-out part.
"The hardest thing for me was I wasn't prepared for how that amount of processed food will affect your mood and I dealt with depression for the first time. And so what they say about what you eat is who you are, it's so true," she explained, "because I ate like a person who just didn't move and I felt like that and I was lethargic and tired all the time. That was a hard thing to break because it's more mental than it is almost physical. You can stop eating, but if you don't feel good about yourself, then it's hard."